Star system

A star system, or stellar system, consists of a small number of stars held together by gravity as they orbit one another. The term can occasionally be used for a single star. Large gravitationally bound groups of stars are usually called star clusters or galaxies, though they are technically star systems as well. Star systems should not be confused with planetary systems, which contain planets, comets, and similar objects.

A system with two stars is called a binary star, binary star system, or physical double star. Systems with four or more stars are rare and much less common than those with two or three. Multiple-star systems are named by number: ternary for three stars, quaternary for four, and so on. These systems are smaller than open star clusters, which have more complex dynamics and typically contain 100 to 1,000 stars.

Physical multiple stars are distinct from optical multiples, which only appear close together from Earth. The term "multiple stars" can refer to either type, but optical multiples do not form a true star system. A triple star that is not fully gravitationally bound might consist of a physical binary and an optical companion, or, rarely, a purely optical triple.

Research suggests that roughly one-third of star systems in the Milky Way are binary or multiple, while two-thirds are single stars. Among non-single systems, binaries are the most common. The number of known systems drops sharply as the number of stars increases. For instance, in a 1999 catalog of physical multiple stars, 551 out of 728 systems were triple, but selection effects make these statistics hard to interpret.

Several methods can detect star systems and distinguish them from optical multiples. Observations taken six months apart can reveal parallax differences, though this is not feasible for distant stars. Direct observation of stars orbiting each other or an empty space (like a dim star or neutron star) is possible but impractical for distant stars or those with long orbital periods. Other methods include measuring varying Doppler shifts, watching for brightness changes from eclipses (which requires Earth to be in the orbital plane), or detecting brightness fluctuations caused by stars reflecting each other's light or gravitationally distorting each other.

In systems that follow the two-body problem—with negligible tidal effects, perturbations, and mass transfer—the two stars trace a stable elliptical orbit around the system's barycenter. Examples include Sirius, Procyon, and Cygnus X-1 (a star and a black hole). Multiple-star systems fall into two dynamical classes. Hierarchical systems are stable, with nested orbits that barely interact, and each level can be treated as a two-body problem. Trapezia have unstable, strongly interacting orbits, behave chaotically as an n-body problem, and can contain two, three, or four stars.

Most multiple-star systems are hierarchical: the stars split into two smaller groups, each orbiting the system's center of mass on a larger path. Each group must itself be hierarchical, dividing into smaller subgroups, and so on. At each level, close pairs can be treated as a single star, with little orbital interaction, so the motion approximates stable Keplerian orbits. For example, a stable triple system has two stars in a close binary, with a third orbiting far away. If the inner and outer orbits are similar in size, the system may become unstable and eject a star. EZ Aquarii is a physical hierarchical triple, with an outer star orbiting an inner binary of two red dwarfs.

Hierarchical arrangements can be illustrated with mobile diagrams, resembling hanging ornaments. Each level shows the system broken into two or more smaller systems. A diagram is called multiplex if a node has more than two children—meaning a subsystem has two or more orbits of comparable size. Because multiplexes can be unstable, multiple stars are expected to be simplex, where each level has exactly two children. The number of levels in the diagram is its hierarchy. A simplex diagram of hierarchy 1 describes a binary. Hierarchy 2 can describe a triple or a quadruple. Hierarchy 3 can describe a system with four to eight components, such as a distant component orbiting a close binary, where one of the close binary's stars is itself an even closer binary. A real example of hierarchy 3 is Castor (Alpha Geminorum).